1. Field of the Invention
This invention relates to an amplitude modulation method and a broadcasting apparatus to which the method is applicable, and more particularly to technologies for simplifying the power amplifier and power supply circuit configuration.
2. Description of the Related Art
A digital modulation system that can be used with a fully solid state radio broadcasting apparatus is described in "Digitally Implemented Medium Frequency Transmitter And Its Application" written by Shinya HIRANO on pages 119-125 of the April 1991 issue of Hoso Gijutsu (Broadcasting Technologies).
In the modulation system, analog-to-digital (A/D) conversion of audio signals of modulation signals is first made by a 12-bit A/D converter to provide 12-bit digital audio signals. The high-order six bits of the digital audio signal are used to control the on-off states of 42 big step modules and the low-order six bits to control the on-off states of six binary modules.
The big step modules and binary modules are power amplifiers which amplify RF carriers. The gains of the six binary modules are 1/2, 1/4, 1/8, 1/16, 1/32, and 1/64 those of the big step modules respectively. RF outputs of the big step modules and binary modules are added together, and the resultant RF signals are transmitted. Therefore, the amplitude of the resultant RF signal can be changed in big steps by selectively turning on and off the big step modules; it can be more finely changed by selectively turning on and off the binary modules. This shows that amplitude-modulated waves can be obtained as the resultant RF signals after combining by controlling the on-off states of the big step modules and binary modules.
Thus, hitherto, a large number of power amplifiers whose on-off states can be controlled have been used to digitally generate amplitude-modulated waves. In the example given above, 42 big step modules and six binary modules, namely, 48 power amplifiers are used in total. The configuration of a broadcasting apparatus is enlarged by using a large number of power amplifiers. Therefore, it is not economical to adopt such a configuration for a transmitter having a comparatively small output capability of about 1 kW.
Further, generally the power supply voltages of power amplifiers must be made different according the output specifications of the power amplifiers. In the example given above, the supply voltage of each big step module is 230V; the supply voltages of the four binary modules having outputs of 1/2, 1/4, 1/8, and 1/16 that of the big step module are each 115V; and the supply voltages of the two binary modules having outputs 1/32 and 1/64 that of the big step module are each 30V. Thus, in the conventional apparatus, a number of voltages must be provided as power amplifier supply voltages and complication of the power circuit configuration is inevitable.
Further, since the modulation system described above adopts uniform quantization as the quantization method, quantizing noise increases rapidly as the amplitude of an audio signal input as a modulation signal becomes small. This causes the S/N ratio to lower and the distortion factor to increase. As shown in FIG. 1, the distortion factor when the degree of modulation is 10% (that is, when the input audio signal amplitude is comparatively small) is about 10 times higher than when the degree of modulation is 100% (that is, when the input audio signal amplitude is comparatively large).